Phase-dependent thermodynamics and kinetics of sulfate radical oxidation of metazachlor herbicide: a theoretical study
摘要
The sulfate radical anion (SO4●–)-initiated oxidation of the herbicide metazachlor (MTZ) was investigated in aqueous and gaseous phases using the density functional theory at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G(d,p) level of theory. Three oxidation mechanisms, including abstraction (Abs), addition (Add), and single electron transfer (SET), were explored to elucidate mechanisms and kinetics. Most oxidation reactions were thermodynamically favorable and spontaneous in Both phases. The overall rate constant at 298.15 K was significantly higher in the gas phase (1.51 × 1013 M−1 s−1) compared to water (4.50 × 1010 M−1 s−1). In water, the degradation process was non-selective, involving various Add and Abs-reactions with similar apparent rate constants kapp ranging from 2.34 × 109 to 2.53 × 109 M−1 s−1 and corresponding branching ratio (Γ) being between 5.20 and 5.63%. In contrast, the gas-phase degradation was highly selective, with the Abs-H24 pathway showing the highest rate (1.08 × 1013 M−1 s−1 of kapp and 71.76% of Γ). Temperature-dependent analysis revealed that reaction rates increased with temperature in water (283–323 K), extending MTZ lifetimes from microseconds to hours, but decreased in the gas phase (253–323 K). Notably, abstraction reactions involving methyl and methylene groups predominantly followed a proton-coupled electron transfer mechanism. Ecotoxicology predictions indicate that degradation products from Add reactions exhibit reduced acute and chronic toxicity, as well as lower bioaccumulation, developmental, and mutagenic potential compared to MTZ. These findings highlight the efficacy and potential of SO4●–—based advanced oxidation processes for MTZ removal across different phases.
Graphical abstractThe SO4●– degradation of metazachlor is highly efficient, with rapid rates in gas and water but follows distinct temperature-dependent mechanisms.